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CHEM2211 > PMcG > Flashcards

PMcG Flashcards

1
Q

Which of the two isoelectronic compounds [Cr(CO)6] and [V(CO)6]− will have the higher CO stretching frequency?

A

The negative charge on the V complex will result in greater π back-bonding to the CO π* orbitals, compared to the Cr complex. This back-bonding results in a weakening of the CO bond, with a corresponding decrease in stretching frequency. Thus the Cr complex has the higher CO stretching frequency.

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2
Q

Which of the two chromium compounds [Cr(CO)5(PEt3)] and [Cr(CO)5(PPh3)] will have the lower CO stretching frequency? Which will have the shorter M–C bond?

A

PEt3 is more basic than PPh3 and thus the PEt3 complex will have greater electron density on the metal atom than the PPh3 complex. The greater electron density will result in greater back-bonding to the carbonyl ligand and thus both a lower CO stretching frequency and a shorter M–C bond.

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3
Q

M-CO bonding

A
  • The 3σ orbital on CO serves as a very weak σ-donor and the π* orbitals act as π-acceptors.
  • CO is good at stabilising low metal ox states.
  • CO stretching frequency is decreased when it serves as a π-acceptor.
  • CO stretching frequencies are often used to determine the order of acceptor/donor strengths of other ligands in a complex: CO stretching frequency increases when there are other π-acceptors and decreases when there are strong donor ligands.
  • When CO is bound to a metal, polarisability at C is reversed; consequentially, C becomes susceptible to nucleophilic attack and O to electrophilic attack.
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4
Q

M-NO bonding

A
  • Not strictly an organometallic.
  • NO can bind to a metal in two different ways, either bent or linear.
  • In the linear arrangement, it is considered NO+, which is isoelectronic to CO.
  • In the bent arrangement, it is considered as NO-.
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5
Q

M-PR3 bonding

A
  • Not strictly organometallic.
  • PR3 bonds to metals by a combination of σ-donation from the P atom and π-back bonding from the metal atom (which enters σ* on P).
  • Analogous to bonding of CO to a d-metal atom.
  • Two important properties: steric-bulk and electron-donating/accepting ability, altered on variation of R-groups.
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6
Q

M-PR3 electronic properties

A
  • The electron withdrawing/donating ability of PR3 R-groups determines the extent of electron withdrawal/donation to the metal.
  • This ability can be measured for different Phosphine ligands using CO stretching frequencies.
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7
Q

M-PR3 steric properties

A
  • Represented by the Tolman cone angle. This angle will tell us how many PR3 ligands can fit around a metal (with varying R-groups).
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8
Q

M-Alkyl bonding

A
  • Monohapto(n1), single-electron donors.
  • Metal-carbon σ-bonding only (no π-interaction).
  • Long alkyl chains that possess a hydrogen atom attached to the carbon adjacent to the carbon-metal have great tendency to decompose by the process of b-elimination.
  • Alkyl groups that cannot react this way are therefore much more stable.
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9
Q

How are M-alkane bonds formed?

A
  • Nuc- attack of M-alkyl on M-X: WCl6 + 6MeLi ¬ WMe6 + 6LiCl.
  • Nuc- attack of M-CO on M-X: Mn(CO)5 + MeI ¬ MeMn(CO)5.
  • Oxidative addition to a 16e- complex: Pt(PPh3)4 + MeI ¬ MePtI(PPh3)2 + 2PPh3.
  • Insertion: PtHCl(Pet3)2 + C2H4 ¬ PtEtCl(PEt3)2.
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10
Q

M-Alkene bonding

A
  • Dihapto(n2), 2-electron, neutral ligands.
  • σ interaction from filled C=C(π) to empty M(σ).
  • In parallel, empty C=C(π*) accepts back-donation from M(π).
  • With stronger back-donation, C=C(sp2) becomes closer to C-C(sp3).
  • Bonding between metal and carbon reverses polarisability at C and makes the alkene susceptible to nucleophilic attack.
  • The less the metal back-donates electron density to the carbon, the more the chemical character of the alkene ‘inverts’.
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11
Q

How are M-alkenes formed?

A
  • First prepared by Zeise (Zeise’s salt): K2[PtCl4] + C2H4 ¬ K[PtCl3(C2H4)]*H2O (dilute HCl conditions).
  • Facilitated by a non-coordinating anion: CpFe(CO)2I + C2H4 + AgBF4 ¬ [CpFe(CO)2(C2H4)]BF4 + AgI.
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12
Q

Hapticity n* (eta)

A
  • Definition: The participation of multiple atoms simultaneously to the bonding between a carbon and metal centre.
  • Example (n2): Here, π-electron density is donated to the metal from two atoms.
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13
Q

M-Di-hydrogen-hydride bonding

A
  • Di-hydrogen donates two electrons from its σ-bond.
  • The σ*-orbital can accept electron density in back-donation from M.
  • Back-donation causes the H-H bond to lengthen and ultimately break, giving a di-hydride complex.
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14
Q

electron counting: neutral atom model

A
  • using the MLXZ system: M=element, L=2e-ligand, X=1e-ligand, Z= empty orbital ligand.
  • metal atom is taken as zero oxidation state for counting purpose.
    -electrons from metal = metal group number
    -class ligands by L,X,Z and add their contribution to metal to get total count.
  • ox state of metal is then taken to be equal to number of X-ligands.
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15
Q

1e- donors

A

-alkyl
-aryl
-H
-halide
-NR2
-OR

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16
Q

2e- donors

A

-CO
-CNR
-alkene
-PR3
-carbene

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17
Q

3e- donors

A

-bridging halide
-n3-ally
-alkyne

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18
Q

4, 5, 6e- donors

A

4: diene
5: n5-Cp
6: n6-arene

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19
Q

Classes of complexes

A

Class(I): does not obey 18e- rule; small oct splitting; t2g is non-bonding; 12-22 VE; includes tet complexes.
Class(II): number of VE does not exceed 18; large oct splitting; typically 4d and 5d metals and high ox states; t2g is non-bonding.
Class(III): number of VE exactly 18; largest oct splitting; applies to good sigma-donors and π-acceptors; t2g is bonding

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20
Q

How are M-cyclopentadienyl (Cp) complexes formed?

A
  1. Metal salt, e.g., MCl2 and Cp reagent, e.g., NaCp.
  2. Metal and Cp
  3. Metal salt and Cyclopentadiene.
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21
Q

M-Cp bonding modes

A

-n1 (L-type).
-n3 (LX-type).
-n5 (LX2-type).
-n5 is the most common.

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22
Q

Nature of M-Cp (n5) bonding

A

-Cp donates four electrons from the π-system to M(σ-type) AO’s. This can be pictured like 2x alkene σ-donation (L2-type).
-Cp donates single e- via σ-interaction with M (X-type).
-Cp π* accepts e- density in back-donation.
-Overall L2X.
- Overall, bonding removes e- density from Cp ring.

23
Q

What are the consequences of M-Cp bonding on Cp reactivity?

A

-Strong electron-donation from Cp renders the M-center electron-rich.
-Increases Cp’s reactivity.
-Makes Cp electrophilic.
-Thus, greater susceptibility to Nuc- aromatic subst.
-Makes Cp 10^6x better Friedel Craft’s reactant than benzene.

24
Q

Ferrocene

A

-Central Fe ‘sandwiched between’ 2 Cp (n5) ligands.
-High stability.
-Fe-Cp bonding makes Cp electrophilic, so susceptible to nucleophilic aromatic substitution.
-An adduct of ferrocene has been made insoluble for use in testing sugar levels, where it acts as an electron shuttle.
- Used extensively in catalysis and as anticancer drugs.

25
Q

Cp ligand properties

A

Cp ligands with different substituents can be complexed to metals, to form complexes with an array of different functionalities: sterically crowded, EDG Cp ligands can effectively stabilise metal complexes. AND the addition of chiral Cp ligands can result in different stereochemical properties.

26
Q

Bent metallocenes

A

-M possesses two σ and one π orbitals which point away from Cp ligands.
-Depending on VE count on M, can either donate (Lewis base) or accept (Lewis acid) π-electron density.

27
Q

M-Arene bonding

A

-Effectively 3x alkene ligands (n6).
-L3-type.
-n2 (L-type) and n4 (L2-type) are known.
-Electron donation is stronger than for CO, but weaker than for Cp.
-M(π) back-bonding is possible, but not very strong.
-Common for M to be lower valent/’soft’.

28
Q

What are the consequences of M-arene bonding on arene’s reactivity?

A

-Makes arene electrophilic.
-Increased susceptibility to nucleophilic aromatic substitution.
-This effect is enhanced when M is electron poor.
-Activation at the ortho-positions of substituted benzene is particularly strong.
-Stabilised carbon nucleophiles will attack Cr-bound benzene rings, which can lead to addition or substitution products.

29
Q

How are M-arene sandwich complexes synthesised?

A
  1. Reduction of M-halide in presence of arene, requiring use of a Lewis acid and mild reducing agent.
  2. Entropically driven synthesis - “co-condensation”.
    -There is a large kinetic barrier to formation, despite the products’ thermodynamic stability.
30
Q

Stability of arene sandwich complexes

A

-The only benzene sandwich complexes stable at room temp are those of Cr, Cr(I), Mo, Mo(I), W, W(I) and Ru(II).
-Sandwich complexes with more sterically hindered arenes are more stable.

31
Q

Cyclobutadiene

A

-Anti-aromatic (4-π e-) compound.
-Alternating C-C and C=C bond-lengths.
-Unstable in free, neutral form.
-Reacts with itself via Diels-Alder reaction above 35K.
-Some free cyclobutadiene compounds with very sterically crowded substituents can be isolated at room temp.

32
Q

M-cyclobutadiene bonding

A

-π back-donation is strong, and stabilises the ligand.
-Behaves more like an arene than free cyclobutadiene.
-Complexed cyclobutadiene is best considered as [C4H4]2- (a 6 π-electron, LX2 ligand).

33
Q

How are M-cyclobutadiene complexes formed?

A

-Owing to cyclobutadiene’s instability in free form, it must be generated in the presence of the metal with which it is to be coordinated to.
-One method is the reaction of a halogenated cyclobutene, e.g., C4H2Cl2, with, e.g., Fe2(CO)9. This results in dehalogenation on the ligand, and coordination to the metal.
-Another method is dimerisation of a substituted ethyne.

34
Q

M-cyclobutadiene behaviour

A

-Behaves more like an arene than free cyclobutadiene.
-Will undergo Friedel-crafts acylation and not the Diels-Alder reaction.

35
Q

M-allyl (n3) bonding

A

-A combination of M-alkene (L-type) and M-alkane (X-type).
-Overall XL.
-3 π-MO’s available on allyl to interact with M.
-Strong π-donation into dz^2 and dxz.
-Weak back bonding into dxy.

36
Q

M-allyl (n1) bonding

A

-Binding is only through a single carbon.
-X-type.
-No back bonding.
-Interconversion between n1 and n3 forms is common.

37
Q

How are M-allyl complexes formed?

A
  1. Salt metathesis using Grignard reagents.
  2. Deprotonation of coordinated propene.
  3. Oxidative addition of allyl halides.
    -Note that it is common for n3 complexes to form via displacement of a ligand on an n1 complex.
38
Q

What are the uses of bound allyl’s?

A

-M-allyl interactions the result in regions of +ve charge on the allyl.
-This renders allyl carbons susceptible to nucleophilic attack (specific to Pd complexes).
-C=C bond migration is made possible when allyl’s are bound to a M center able to donate 2 electrons.

39
Q

Define a ‘soft’ metal and state how bound allyl’s will react

A

-Late TM’s.
-VE’s are far from the nucleus.
-M-allyl is strongly covalent.
-C1 and C3 hold delta+, and are susceptible to nucleophilic attack.

40
Q

Define a ‘hard’ metal and state how bound allyl’s will react

A

-Early TM’s.
-VE’s are close to the nucleus.
-M-allyl is more ionic.
-C2 holds delta+, and is susceptible to nucleophilic attack.

41
Q

Consequences of nucleophilic attack on Pd-allyl

A

-The reaction is specific to Pd complexes.
-Pd is reduced.
-If Pd is cationic (M+), this helps the process of nucleophilic attack, as Pd+ further removes electron density from allyl, making allyl more electrophilic - metal-promoted elimination.
-Process is formally an allylic substitution reaction.

42
Q

M-allyl double-bond migration reactions

A
  1. M coordinates to C=C in L-type fashion.
  2. Oxidative addition using neighbouring C-H, resulting in M oxidation and allyl (n3, LX-type coordination).
  3. Reductive elimination at the opposite C atom, resulting in L-type M-alkene bonding, with C=C having migrated.
    -These three steps repeat until C=C is in most favourable position.
    -Not specific to Pd, but possible for any M that can undergo 2-electron OA and RE.
43
Q

Other M-enyl complexes

A

-Enyl ligands are alkenyls containing an odd number of carbons.
-XLn ligands.
-The delocalised component is planar.

44
Q

How are M-enyl complexes formed?

A
  1. Electrophilic attack at a neutral alkene precursor: by either protonation or hydride (H-) extraction.
  2. Nucleophilic attack at a neutral alkene precursor: NaOMe can act as nucleophile. Nucleophile will always add on opposite face to metal.
  3. Can be made by salt metathesis if the parent alkene is stable in deprotonated form.
45
Q

How do M-enyl complexes react?

A

-For cationic enyl complexes, the enyl is activated towards nucleophilic attack (as for allyl’s).
-Attack occurs on terminal carbon (as for allyl’s).
-Fe is commonly used because it is cheap and accessible. Pd is often used also.
-Following nucleophilic attack, an oxidative work-up removes M fragment.

46
Q

Schrock-type carbenes

A

-Triplet carbenes.
-X2 ligand, with unpaired electrons in both an sp2- and a p-orbital.
-σ- and π- donation.
-Carbonyl donates more electron density to M than it receives.
-R = π-EWG/σ-EDG, H or alkyl/aryl.
-Bonds to high ox state, electron poor M.

47
Q

Fischer-type carbenes.

A

-Singlet carbenes.
-L ligand, with filled sp2 orbital for donation.
-σ-donation.
-Empty p-orbitals can accept back donation.
-R = π-EDG/σ-EWG, heteroatom e.g., OR or NR2.
-Strong π-EDG groups weaken back donation.
-Bonds to low ox state, electron-rich M.

48
Q

Important applications of Schrock carbenes in organic synthesis

A

-Alkene metathesis.
-Involves the exchange of a CR2 fragment between an alkene and M.
-Involves electrophilic attack on the alkene reagent at M=C bond in a 2+2 cycloaddition.
-A powerful way to construct substituted alkenes during a synthesis or polymerisation reaction.
-Fischer carbenes are not electrophilic enough for this.

49
Q

Carbyne complexes

A

-More similar to Schrock than Fischer carbenes.
-Rare for carbynes to have heteroatom substituents.
-sp-hybridised, X3 ligands.
-The third component of bonding involves a second M t2g orbital.
-M-C bond is shorter and bond-angle wider than for carbenes.

50
Q

NHC ligands

A

-Diaminocarbenes.
-Typically 5-membered rings that incorporate N atoms adjacent to carbene C.
-Sp2-hybridised.
-Singlet state dominates (both e-‘s in hybridised AO, available for dative bonding to M).
-Overlap of filled N p-orbitals with C’s empty p-orbitals stabilises carbene.
-Carbene C is highly reactive and susceptible to dimerisation. In order to isolate ‘free’ NHC, bulky substituents are needed to ‘protect’ C from reactivity.
-Highly useful ligands in organometallic chemistry, and often compared to PR3’s.

51
Q

Usefulness of NHC’s as ligands

A

-Strong σ-donor ‘L’ ligands.
-Exert a steric influence about M coordination sphere.
-Steric properties easily varied by changing N substituents.
-Potential for back bonding.

52
Q

M-NHC bonding

A

-L ligand, with strong σ-donation.
-C p-orbitals are conjugated to N p-orbitals so not really ‘empty’, therefore back donation is not so strong.
-Back donation is stronger where M is electron-rich.

53
Q

Applications of M-NHC complexes

A

-Catalysis: Cross-coupling, olefin metathesis, C-H bond activation, olefin oligomerisation and polymerisation.
-Biomedical: Antimicrobial agents, anticancer agents.

54
Q

Grubb’s 2nd gen metathesis catalyst

A

-Ru complex
-PR3 ligand replaced by NHC.
-Significantly higher activity in olefin metathesis.
-Stronger trans effect of the NHC promotes phosphine dissociation.
-Greater affinity of 14e- species to bind alkene substrates.

CHEM2211 (3 decks)

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